A major user of silica sol is the steel industry since the steel mills use sols as a binder in the makeup of the stools on which a hot ingot cools. The sol acts as a thermal insulator, or an anti-bonding agent, to keep the hot ingot from sticking to the stool. Another major user is the investment casting industry wherein the investment caster uses the sol as a binder for very high precision casting molds. Other interesting side uses for silica sol are in the floor polish and toothpaste industries. In floor polish, sols act as anti-slip agents. Also because the particles are very small, there is virtually no light scattering, and the polish is clear. In toothpaste, colloidal silica, in the form of silica gel, is used as an abrasive. The silica particles are much smaller than the normally used phosphate particles and this results in less tooth enamel abrasion.
Various techniques for producing silica sols are known to the art. For example, U.S. Pat. No. 1,562,940 concerns a process for producing silicic acid electrolyzing an aqueous solution of water glass between an anode and a mercury cathode while continuously agitating the anode by having means affixed thereto to effect its rotation.
In U.S. Pat. Nos. 3,654,105 and 3,654,865, general techniques for producing silica sols are disclosed which include the step of passing an electric current between an anode and a cathode which are positioned in a specified electrolyte with the anode being fabricated from silicon or a silicon containing material.
U.S. Pat. No. 3,668,088 discloses an electrodialysis process for making silica sols. In this electrodialysis process, a sol-electrolyte containing an aqueous sodium silicate solution is electrolyzed while separated from an acid anolyte by a cation-permeable, anion-impermeable membrane. Hydrogen ions from the anolyte are drawn through the membrane into the sol-electrolyte and sodium ions are removed from the electrolyte by bringing them into contact with a mercury cathode so as to form a mercury-sodium amalgam. The silica from the sodium silicate deposits on the silica particles present in the sol-electrolyte. U.S. Pat. No. 3,723,273 discloses the electrodialytic production of stannic oxide sols with a cation permselective membrane.
In commonly assigned U.S. patent application Ser. No. 669,188 filed Mar. 21, 1976 now abandoned, there is described a method of producing a silica sol by electrodialytically transferring alkali metal cations of a water-soluble alkali metal silicate from an anode compartment of an electrolytic cell to a cathode compartment of the electrolytic cell while essentially preventing the migration of silica into the cathode compartment by positioning a cation permselective membrane between the anode compartment and the cathode compartment, and removing the so-produced silica sol from the anode compartment of the electrolytic cell. Deposition of silica on the anode is avoided by careful control of the pH of the anolyte at a value from about 7.5 to about 8.5, preferably 8 to 8.2.